Nuclear medicine is a field of medicine concerned with the use of radiation for diagnostic purposes. Single Photon Emission Computed Tomography (referred to in this specification as “SPECT”), a branch of nuclear medicine, involves directly measuring gamma rays emitted by radionuclides administered to a patient to produce slice-like images of the patient. “Tomography” refers to the production of slice-like images, or tomograms. Computerized Tomography (CT) refers to the use of computer processing to derive the tomogram.
Typically, in SPECT procedures, radiopharmaceuticals (otherwise known as radioactive tracers or radiotracers) are administered to patients. Radiopharmaceuticals are generally compounds consisting of radionuclides (i.e. radiation-emitting atoms), combined with pharmaceuticals or other chemical compounds. In some cases, such as with Thallium-201, the same particle is simultaneously the radionuclide and pharmaceutical. Unlike positron emission tomography (PET) which uses small radionuclides with half-lives of just over a minute to under 2 hours, SPECT involves the use of radionuclides whose half-life is several hours to days long, long enough to clinically localize or become fixed in specific organs or cellular receptors. In these circumstances, it is possible to acquire important diagnostic information by obtaining images created from the radiation emitted by the radiopharmaceutical.
One frequently-performed SPECT diagnostic procedure is myocardial perfusion imaging (MPI). Approximately ten million such scans are performed in the U.S. annually. For MPI, the patient is injected with a radioactive tracer which collects in, and becomes fixed in, the heart muscle. The localization of the tracer within the heart is dictated by the blood flow through the coronary arteries. These are the blood vessels that supply blood directly to the heart muscle, the myocardium. Therefore, MPI provides important information about the blood flow through the coronary arteries to the heart muscle. Thus, MPI can be used to diagnose serious and potentially fatal heart conditions such as coronary artery disease.
A typical MPI procedure involves a rest, or baseline scan and a stress scan, commonly on the same day, leading with the rest scan. These two scans are employed because important information is provided by the difference in blood flow demonstrated by the two scans. For example, certain coronary arteries of a particular patient may be partially occluded. However, at rest, the coronary artery has the capacity to dilate to offset the occlusion, and a rest scan alone may not show any decreased blood flow through the heart.
On the other hand, under stress, the coronary arteries will dilate to their maximum extent to handle the increased blood flow. Under such conditions, dilation of the diseased coronary artery will not be enough to offset the partial occlusion, with the result that the decreased blood flow will become apparent from the stress scan.
In addition to being used to look for heart defects reflected by anomalies in blood distributions throughout the heart muscle, MPI is used for obtaining information about heart pumping ability, preferably including analyzing heart wall motion and left ventricle ejection fraction. Regarding wall motion, MPI can be used to determine whether there are any abnormalities in the motion of the heart walls, and in particular, the motion of the walls of the left ventricle.
Left ventricle ejection fraction is a term that describes the percentage of blood in the left ventricular cavity that is ejected when the left ventricle contracts to pump blood out of the heart to the body. An ejection fraction that is too low can be indicative of heart disease. Also, the ejection fraction's actually dropping from the rest scan to the stress scan can be indicative of serious heart disease.
For the MPI stress scan, stress is usually created either through exercise, or through the administration of a stress pharmaceutical. A typical order of events when a patient undergoes an MPI scan is as follows. The patient is injected with a radioactive tracer while sitting at rest. Approximately 30-45 minutes later, the patient undergoes the MPI rest scan. The 30-45 minute waiting time has traditionally been needed because some of the tracer typically accumulates in the liver and bowels, and the radiation from that accumulation has been understood to interfere with scanning of the heart. The delay was needed for the tracer in the liver and bowels to dissipate. The rest scan provides information about the resting blood flow within the heart muscle, the resting size of the left ventricle, and the resting left ventricle ejection fraction.
Then, the patient leaves the scanning area to be stressed, returning later for the stress scan.
If the stress is induced by exercise, the patient runs on the treadmill or does some other exercise adequate to produce the required stress. If a stress pharmaceutical is used, then the patient receives the stress pharmaceutical, usually lying down, but sometimes sitting, or even walking on a treadmill. A common stress pharmaceutical is dipyridamole, which is typically infused over about four minutes. Next, at either peak exercise, or at the time of maximal effect of the stress pharmaceutical, the radioactive tracer is administered, and becomes fixed in the heart according to the distribution of the blood flow at the time of the injection.
Typically, the patient returns at least 30-45 minutes after the administration of the stress radioactive tracer in order to be scanned. In practice, the delay is often 2-4 hours because of scanning backlogs that develop through the day. The stress scan reflects relative myocardial blood flow distribution that prevailed at peak stress, that is, at the time of the stress injection. The same scan is used to assess the size and pumping ability of the heart after stress. The stress scan is compared with the rest scan to determine whether there are stress-induced blood flow defects in the heart muscle, whether the heart becomes dilated, and whether it has developed reduced pumping ability, indicated by either a drop in ejection fraction, or visually assessed local wall motion abnormalities.
A problem with this typical protocol is that the information provided after the stress scan, in relation to heart size and pumping ability, is unreliable. Specifically, the stress scan is typically conducted well after the actual peak stress on the heart. There is plenty of time available for the heart to recover and return to its normal size and pumping ability. This delay does not necessarily adversely affect the acquisition of information regarding myocardial blood perfusion in the heart, because the radioactive tracer remains fixed in the heart according to blood flow through the heart during peak stress. However, because the stress scan takes place at least 30-45 minutes after peak stress, but usually after a few hours, it discloses heart size, wall motion and ejection fraction of the heart as it is at least 30-45 minutes after stress. The result is that there are many patients who have enlarged hearts or reduced pumping ability when their hearts are under stress, but whose heart recovers within 30-45 minutes or more, and whose conditions are therefore not detected.